1. Field of the Invention
The present invention relates to a power conversion apparatus for converting DC power into AC power or AC power to DC power, and an electrical-mechanical energy conversion system using this power conversion apparatus.
2. Description of Related Art
The preface of Non Patent Literature 1 (Makoto Hagiwara, Kazutoshi Nishimura, Hirofumi Akagi, “A Medium-Voltage Motor Drive with a Modular Multilevel PWM Inverter Part I. Experimental Verification by a400-V, 15-kW Downscaled Model”, IEEJ Transactionson Industry Applications, April 2010, Vol. 130, No. 4, pp. 544-551) states that “In this paper, the feasibility of a medium-voltage motor drive using a three-phase MMI is considered. A downscaled model of 400V, 15 kW is designed and built, and the control method and operation characteristics will be verified.” FIG. 1(a) of Non Patent Literature 1 shows a main circuit structure of the three-phase MMI. The MMI is an abbreviation of Modular Multi-level PWM (Pulse Width Modulation) Inverter. The three-phase MMI is a power conversion apparatus in which series circuits each including an arm of a series circuit of one or more unit converters and a reactor are connected in three phase bridge configuration. FIGS. 3 to 5 of Non Patent Literature 1 show experimental results obtained by driving an AC motor connected to the AC output terminal of the three-phase MMI.
The three-phase MMI is a kind of multi-level converter and uses a switching element capable of controlling ON/OFF, such as an IGBT (Insulated-Gate Bipolar Transistor), a GTO (Gate Turn-Off Thyristor) or a GCT (Gate-Commutated Thyristor) and can output a voltage higher than the withstand voltage of the switching element.
The power conversion apparatus shown in FIG. 1 (a) of Non Patent Literature 1 requires one reactor for each phase in order to suppress a current circulating through each arm. Thus, the volume and weight of the whole power conversion apparatus may be large, and the installation footprint also may be large.
Further, since the reactor is provided between the arm of each phase and the rotary electric machine, the arm voltage can not be directly applied to the rotary electric machine, and there is the likelihood that the controllability of the rotary electric machine is reduced.